AM radio signals are used in a variety of applications, including broadcast, non-directional navigation beacons, citizens-band radios, and aircraft communication. Various other radio signals with more complex modulations have time-varying amplitudes (envelopes) and can be regarded as having simultaneous amplitude and phase modulation. Examples of complex modulations include Single Sideband (SSB), Independent Sideband (ISB), Vestigial Sideband (VSB), multitone data, multiple carriers amplified simultaneously, and modem shaped-pulse digital-data modulation. Variable-amplitude radio signals are also required in applications such as magnetic-resonance imaging (MRI) and industrial-scientific-medical (ISM) devices.
AM transmitters can be implemented by a variety of techniques (see H. L. Krauss, C. W. Bostian, and F. H. Raab, "Solid State Radio Engineering" Chapter 15, New York, Wiley, 1980), but high-level amplitude modulation is widely regarded as preferable for both quality and efficiency. In high-level amplitude modulation, the main DC supply-voltage input to the final RF power amplifier is varied in proportion to the desired signal amplitude. The RF amplifier is operated in or close to saturation (i.e., at the top of or above its linear operating region). The amplitude (envelope) of the RF output is thereby caused to vary with the supply-voltage input. Throughout this specification and the appended claims, the terminology "high-level modulation," "high-level modulator," etc. refers to such modulation of the main DC supply-voltage input to the final RF power amplifier. It is worth noting that the terms "drain bias" or "collector bias" are sometimes used to refer to a supply-voltage input, especially in microwave engineering. In the present specification and appended claims, the term "supply-voltage input" is meant to include these connection points and any other kind of connection at which the supply voltage enters an amplifier.
High-level amplitude modulation can be used with more complex signals such as SSB through the Kahn Envelope-Elimination-and-Restoration (EER) technique (see L. R. Kahn, "Single Sideband Transmission by Envelope Elimination and Restoration," Proc. IRE, vol. 40, no. 7, pp. 803-806, July 1952). In the classical form of a Kahn-technique transmitter, a limiter eliminates the envelope, producing a constant-amplitude, phase-modulated carrier which becomes the drive to the final amplifier. The detected envelope is amplified by an audio-frequency power amplifier. Amplitude modulation of the final RF power amplifier restores the envelope to the phase-modulated carrier, creating an amplified replica of the input signal. In a modern implementation, the envelope and phase-modulated carrier are produced by a combination of digital signal processing and synthesis.
High efficiency is needed for a variety of reasons. In high-power broadcast transmitters, efficiency determines the consumption of prime AC power and therefore the operating cost. In space-borne and portable transmitters, efficiency determines the size of the battery, power supply, and heat sink. Hence, highly efficient transmitters can be made much smaller and lighter than conventional transmitters. In all cases, improving efficiency reduces the heat dissipated in the RF-power devices, and the resultant lower temperatures increase reliability.
Efficiency can be improved by using a high-efficiency RF power amplifier, a high-efficiency modulator, and a technique such as Kahn EER. However, a limitation on efficiency for low signal levels remains. Often, transmitters must produce low-amplitude signals for a significant portion of the time; hence the efficiency in producing these signals dominates the overall average efficiency.
Drive power is a significant contributor to inefficiency when the transmitter is producing a low-level output. It is well known that the drive (for ideal power amplifiers) can be made to vary with the envelope of the output signal. However, in most real RF-power devices, the gain decreases at lower supply-voltage inputs, which causes them to cease amplification. Furthermore, efficient modulators such as class S modulators work best with fixed, known loads and behave erratically if their load (the RF power amplifier) ceases to draw current.
A detailed discussion of the impact of signal characteristics upon the average efficiency of power amplifiers is given by F. H. Raab, "Average Efficiency of Power Amplifiers," Proc. RF TECHNOLOGY EXPO '86, Anaheim, Calif., pp. 474-486, Jan. 30-Feb. 1, 1986. The instantaneous efficiency (See FIG. 1) of an ideal class-A power amplifier increases with the square of its output voltage, reaching 50 percent at peak-envelope-power (PEP) output. The efficiency of an ideal class-B power amplifier increases linearly with the output voltage to 78.5 percent (=.pi./4) at PEP (see H. L. Krauss, C. W. Bostian, and F. H. Raab, "Solid State Radio Engineering," Chapter 12, New York, Wiley, 1980). In practice, losses in MOSFETs due to resistance reduce the efficiency by 10 to 20 percent, resulting in maximum instantaneous efficiencies of about 40 and 60 percent for class-A and -B power amplifiers, respectively. The presence of load reactance degrades the efficiency even further.
The efficiency of switching-mode power amplifiers (classes D, E, and F) as well as class-C power amplifiers is generally higher than that of a linear power amplifier (class A or B). Because variation of the output amplitude is achieved through variation of the DC supply voltage, the instantaneous efficiency of these power amplifiers remains high for all signal levels. Given proper drive, the efficiency of a class-D power amplifier is subjected to only minor degradation by a reactive load.
Class-D power amplifiers typically achieve Peak Envelope Power (PEP) efficiencies from 75 to 90 percent. For power amplifiers that use Bipolar Junction Transistors (BJTs), the efficiency decreases at lower signal amplitudes because the BJT saturation voltage becomes a more significant fraction of the supply-voltage input. However, for power amplifiers that use MOSFETs, the instantaneous efficiency is largely independent of the output voltage. Saturated power amplifiers of any class generally maintain relatively constant efficiency near the value for peak output.
Continuous Wave (CW) and Frequency Modulation (FM) signals are characterized by constant envelopes and therefore are always at PEP. In contrast, SSB-voice, multitone-data, noise, and shaped-pulse data signals have time-varying envelopes with significant peak-to-average ratios .xi. (typically 6-10 dB).
The probability-density functions (PDFs) of FIG. 2 represent the relative amounts of time that the envelope spends in the vicinity of the corresponding output voltage. The Rayleigh PDF is produced by noise or a multitone signal, while the Laplacian PDF is produced by SSB speech. The PDF of square-root raised-cosine offset quadrature-amplitude modulation (SRRC DQAM in FIG. 2) is typical of that of most modern shaped-pulse digital modulations (See L. Sundstrom, "The Effect Of Quantization In A Digital Signal Component Separator For LINC Transmitters," IEEE Trans. Veh. Technol., vol. 45, no. 2, pp. 346-352, May 1996).
Upon comparison of the instantaneous-efficiency and PDF curves in FIG. 2, it is immediately apparent that the instantaneous efficiencies differ greatly at the signal amplitudes that are most prevalent in real amplitude-modulated signals. To compare different amplifiers with different signals, it is useful to define the average efficiency as ##EQU1##
where P.sub.oAVG and P.sub.iAVG are the average power output and input, respectively.
The average-efficiency characteristics for ideal class-A and -B power amplifiers may be evaluated for a variety of commonly used signals. For a Rayleigh envelope with .xi.=10 dB, the average efficiencies of class-A and -B power amplifiers are only 5 percent and 28 percent, respectively.
Operation of transmitters at less than full power is required for a variety of purposes such as reducing interference and reducing power consumption. Such operation in back-off shifts the PDF curves leftward in FIG. 2 and causes conventional power amplifiers to be even less efficient.
The RF-power amplifiers discussed may employ one or more RF-power devices. RF-power devices, including vacuum tubes, BJTs, MOSFETs, MESFETs, HBTs, HEMTs, HFETs, and pHEMTs, and new devices are continually being developed. Different devices are preferred for different frequencies, power levels, and classes of operation.
The transmitter architecture and types of power amplifier that can be used depend upon the type of signal to be amplified.
FIG. 3 shows the amplifier schematic for a conventional linear architecture transmitter. For the amplifier shown in FIG. 3, the signal becomes progressively larger in each stage of amplification. RF input 10 is coupled to driver amplifier 12 which is coupled to final amplifier 14 which in turn is coupled to RF output 16. Amplifiers 12 and 14 may comprise any type of amplifier, such as class A, B, C, D, E, or F amplifiers. Input signal 13 is applied to RF input 10. Driver 12 produces intermediate signal 15 and final amplifier 14 produces output signal 17 on RF output 16. This type of amplifier can be used to produce both wideband (e.g., audio, pulses, multiple carriers) and narrowband signals (e.g., SSB, TV, FDM, data with shaped envelope). Here, the terms "narrowband" and "wideband" are being used, not to refer to the actual absolute bandwidths of the signals, but in a relative sense. Thus, "narrowband" is used to mean an RF signal whose bandwidth is small (say one-half or less) of its center or carrier frequency; "wideband" is used to refer to other signals that are not so easily characterized as modulation of a carrier. Included in the "linear architecture" category are mildly nonlinear amplifiers that are linearized by techniques such as pre-distortion or feedback so that overall they function as linear amplifiers. Because the linear power amplifiers are inefficient for low-level signals, linear transmitters are inherently inefficient for signals with time-varying envelopes.
Traditional CW, FM, Frequency-Shift Keying (FSK), and Phase-Shift Keying (PSK) signals have constant envelopes and therefore can be produced by nonlinear amplifiers that offer higher efficiency. FIG. 4 shows the amplifier schematic for a conventional nonlinear architecture transmitter. For the amplifier shown in FIG. 4, the signal becomes progressively larger in each stage of amplification. RF input 20 is coupled to driver amplifier 22 which is coupled to final amplifier 24 which in turn is coupled to RF output 26. Amplifiers 22 and 24 may comprise any type of amplifier, such as class A, B, C, D, E, or F amplifiers. Input signal 23 is applied to RF input 20. Driver 22 produces intermediate signal 25 and final amplifier 24 produces output signal 27 on RF output 26. Nonlinear transmitter architectures require output filters; hence, nonlinear transmitter architectures are best suited for narrowband signals.
Amplitude-modulated signals are preferentially produced by the conventional high-level modulation technique shown in FIG. 5. Adder circuit 31 having audio frequency input 29A and carrier level input 29B is coupled to high power amplifier 32 which is connected to the supply-voltage input of RF-power amplifier 37. RF-power amplifier 37 has RF input 36 for receiving RF input signal 34 and has RF output 38. Audio frequency signal 30A is superimposed on carrier signal 30B by adder circuit 31 to set the carrier level for amplifier 32. The output of amplifier 32 is voltage signal 33 in response to which amplifier 37 converts input RF signal 34 to output RF signal 39 which is presented at RF output 38. Amplifier 32 may be a class S amplifier. This architecture is, however, incapable of producing complex signals such as SSB.
The conventional Kahn-technique transmitter architecture, shown in its simplest form in FIG. 6, is based upon the representation of a narrowband signal as simultaneous amplitude and phase modulation. RF input 40 is coupled to envelope detector 41 and limiter 43. Envelope detector 41 is coupled to amplitude modulator 42 the output of which is connected to the supply-voltage input of RF amplifier 44. The output of limiter 43 is coupled to RF amplifier 44, RF amplifier 44 having RF output 45. Input RF signal 46 is received at RF input 40 and converted to phase-modulated RF carrier signal 43A by limiter 43 and simultaneously converted to audio-like envelope signal 41A by envelope detector 41. Envelope signal 41A is applied to amplitude modulator 42. The output of amplifier 42 (amplitude modulator) is voltage signal 42A in response to which RF amplifier 44 converts phase-modulated carrier signal 43A to RF output signal 48, which is presented at RF output terminal 45. RF output signal 48 is an amplified replica of the original signal RF input signal 46. Amplifier 42 may be a class S amplifier. This architecture can produce complex signals and allows the use of efficient but nonlinear RF power amplifiers (classes C, D, E, or F) and the use of efficient modulators (classes G or S) in place of less efficient linear RF power amplifiers and modulators. When the RF bandwidth exceeds the capabilities of an efficient modulator, the Meinzer split-band technique can be used to obtain larger bandwidths with efficiency that is larger than that of a linear modulator (see K. Meinzer, "A Linear Transponder For Amateur Radio Satellites," VHF Communications, vol. 7, pp. 42-57, January 1975).
A conventional Kahn-technique transmitter employing an analog signal source and frequency translation is shown in FIG. 7. Audio frequency input 50 is coupled to SSB modulator 51. The output of SSB modulator 51 is coupled to both envelope detector 52 and delay module 55. Optionally, delay module 55 may be placed after limiter 56, or anywhere else in the RF path ahead of final amplifier 61. Envelope detector 52 is coupled to amplifier 53 having supply-voltage input 64. Amplifier 53 is coupled to first filter 54. First filter 54 is connected to the supply-voltage input of final amplifier 61. Delay module 55 is coupled to limiter 56 which is coupled to second filter 59 through mixer 57. Frequency source (LO) 58 is also coupled to mixer 57. Second filter 59 is coupled to driver 60 having supply-voltage input 64, and driver 60 in turn is coupled to final amplifier 61. Final amplifier is coupled to third filter 62 having an RF output 63. Amplifier 53 may be a class S amplifier and amplifiers 60 and 61 may be class D amplifiers. Filter 54 may be a low-pass filter and filters 59 and 62 may be bandpass filters. Limiting and envelope detection are accomplished at a conveniently low intermediate frequency to obtain high linearity and low amplitude-to-phase conversion. The frequency-conversion process is the same as that in other transmitters, and multiple stages of frequency conversion may be used to ensure low levels of spurious products. Delay module 55 matches the delay of the phase-modulated information to the delay introduced in the amplitude modulator.
High efficiency can be achieved by using nonlinear but efficient power amplifiers in place of the linear power amplifiers used in other architectures. Nonlinear RF power amplifiers, including classes C, D, E, and F, offer better efficiency than do linear power amplifiers (classes A and B). Linear power amplifiers can also be operated at or near saturation to obtain their maximum possible efficiency. High-efficiency high-level amplitude modulation is accomplished by class-S, class-G, or pulse-step-modulated audio frequency power amplifiers. (Hybrid combinations, such as combining a class-S with a linear amplifier, e.g. class A or B, could also be used.) Class-S amplifiers are basically similar in operation to switching-type voltage regulators or "buck" converters.
While conventional linear transmitters suffer from poor efficiency for low-amplitude signals, Kahn-technique transmitters have good efficiency over a wide dynamic range.
A Kahn-technique transmitter that operates at High Frequency (HF) and Very High Frequency (VHF) and employs a class-D RF power amplifier and a class-S modulator is described by the inventor in papers by F. H. Raab and D. J. Rupp, "High-Efficiency Single-Sideband HF/VHF Transmitter Based Upon Envelope Elimination And Restoration," Proc. Sixth Int. Conf. HF Radio Systems and Techniques (HF '94) (IEE CP 392), York, UK, pp. 21-25, Jul. 4-7, 1994, and by F. H. Raab and D. J. Rupp, "High-efficiency Multimode HF/VHF Transmitter for Communication and Jamming," Proc. MILCOM '94, Ft. Monmouth, N.J., pp. 880-884, Oct. 2-5, 1994. Its RF-power amplifier is described in the paper by F. H. Raab and D. J. Rupp, "HF Power Amplifier Operates In Both Class B And Class D," Proc. RF EXPO WEST '93, San Jose, Calif., pp. 114-124, Mar. 17-19, 1993. Its modulator is described in the paper by F. H. Raab and D. J. Rupp, "Class-S High-Efficiency Amplitude Modulator," RF DESIGN, vol. 17, no. 5, pp. 70-74, May 1994 and in the paper by F. H. Raab and D. J. Rupp, "High-Efficiency Amplitude Modulator," Proc. RF EXPO EAST '94, Orlando, Fla., pp. 1-9, Nov. 15-17, 1994. It is expected that an L-band Kahn-technique transmitter that uses a conventional power amplifier operated in saturation will be described in a presentation by F. H. Raab, B. E. Sigmon, R. G. Myers, and R. M. Jackson, "High-Efficiency L-Band Kahn-Technique Transmitter," INT. MICROWAVE SYMPOSIUM, Baltimore, Md., Jun. 7-12, 1998.
A variation on the Kahn EER technique called "envelope tracking" allows the RF-power amplifier to be operated in its linear region with improved efficiency. It is useful when saturation of the RF-power amplifier causes undesired effects such as amplitude-to-phase conversion. The architecture is similar to that of FIG. 6 or 7 with the limiter removed. The gain of the class-S amplifier is set to produce a slightly higher supply voltage than is actually needed to support the current RF output. This can be done continuously (by increased gain or voltage offset) or in steps (as described in the paper by F. H. Raab entitled "Efficiency of envelope-tracking RF power-amplifier systems," Proc. RF EXPO EAST '86, Boston, Mass., pp. 303-311, Nov. 10-12, 1986) or continuously. There is no specific requirement for operation close to saturation; however, efficiency increases with proximity to saturation.
In many applications for full-carrier amplitude modulation, the output amplitude does not actually drop to zero. For complex modulations, however, the output envelope almost always goes through zero; hence, the transmitter for these signals must be capable of linear operation at all amplitudes from zero to peak output.
The conventional Kahn-technique transmitter shown in FIG. 7 employs a constant-amplitude driving signal derived from the hard-limited carrier. As a result, the power consumed by the driver is constant. When the transmitter is operating at peak output, the constant drive power causes only a small reduction in transmitter efficiency. At lower output power, however, the drive power is a much larger fraction of the transmitter power, resulting in a greatly reduced transmitter efficiency (see FIG. 8). This results in inefficient amplification of signals with large peak-to-average ratios.
A second disadvantage of constant-amplitude drive is a relatively large feed-through signal that appears as distortion in the transmitter output. The gate-drain (or base-collector) capacitance in the final amplifier couples some of the drive signal to the transmitter output. Since the drive signal is hard-limited, it has sidebands that cause intermodulation distortion (IMD) when coupled to the output.
Drive power can, in principle, be reduced by modulating the supply-voltage input to the driver amplifier. The same class-S modulator can be used as when both final and driver amplifiers operate from the same supply-voltage input. Such a Kahn-technique transmitter is shown in FIG. 9. Audio frequency input 70 is coupled to SSB modulator 71. The output of an SSB modulator is coupled to both envelope detector 72 and delay module 75. Envelope detector 72 is coupled to amplifier 73 having supply-voltage input 84. Amplifier 73 is coupled to first filter 74. First filter 74 is connected to the supply-voltage input of final amplifier 81 and the supply-voltage input of driver 80. Delay module 75 is coupled to limiter 76 which is coupled to second filter 79 through mixer 77. Frequency source (LO) 78 is also coupled to mixer 77. Second filter 79 is coupled to driver 80; driver 80 in turn is coupled to final amplifier 81. Final amplifier 81 is coupled to third filter 82 having an RF output 83. Amplifier 73 may be a class S amplifier and amplifiers 80 and 81 may be class D amplifiers. Filter 74 may be a low pass filter and filters 79 and 82 may be bandpass filters. Drive power varies in direct proportion to the output power, resulting in a constant transmitter efficiency at all output levels (see FIG. 8). The level of drive feed-through is reduced, and the unwanted sidebands of the drive signal are eliminated. This technique is known in the art, having been used with high-level modulation of AM transmitters with class-C RF power amplifiers, and having been taught by the inventor in seminars on high-efficiency power amplifiers, for example in a paper entitled "Envelope Elimination And Restoration And Related Feedback Systems," Research Note RN86-34 (Rev. B), Green Mountain Radio Research Company, Winooski, Vt., Dec. 8, 1987.
Unfortunately, the gain of virtually all RF-power devices (for example, transistors such as BJTs, MOSFETs, and GaAsFETs) decreases at lower signal levels. As a result, at the lower signal levels, the drive becomes insufficient to produce the desired output. This causes reduced output or no output, with attendant distortion of the signal. In this condition, the final amplifier no longer loads the class-S modulator (for example) as expected. The capacitors in the output filter of the class-S modulator fail to discharge, resulting in distortion of the final-amplifier modulating waveform. Similar effects can occur with modulators other than class-S modulators.
The resultant low-signal drop-out and distortion can be seen in the transfer curve of FIG. 10, the envelope waveforms of FIG. 11, and the output spectrum of FIG. 12, measured in an experimental transmitter.